With explosively increasing of services in the communication industry, the requirement for communication capacity, flexibility and quality continuously increases. Technology develops from analog to digital, from FDMA (Frequency Division Multiple Access) to TDMA (Time Division Multiple Access) and to existing CDMA. Compared with a generally used TDMA system, CDMA has some special advantages, such as higher efficiency of frequency use, larger communication capacity, better security, better service and lower cost. Therefore, DS-CDMA (Direct Sequence Code Division Multiple Access) system gradually becomes a necessary selection to meet the requirement of rapid increasing communication in a third generation of mobile communication system. The main difference between DS-CDMA and TDMA or FDMA systems is that in a DS-CDMA system, all subscribers use the same frequency bandwidth, and different subscribers are distinguished by a uniquely assigned PN code sequence. The invention is suitable for an application using a protocol standard of the third generation of mobile communication based on DS-CDMA technology.
It is noted that rapid channel fading is an outstanding problem in digital mobile communication. In a process of signal transmission, a transmission channel is severely affected by fading caused by selection of time and frequency. It is necessary to develop advanced and reliable channel detection and estimation technology to perform accurate estimation to balance or correct channel distortion, so as to reliably recover or restore a transmit signal at a receiver. Also, advanced and reliable channel detection and estimation technology can alleviate the limitation of performance of the other parts in a communication system to a great extent, thereby developing a system with larger capacity, smaller size and lower cost. For example, a base station with excellent receiver performance allows a mobile station to use a smaller transmit power so as to reduce battery consumption at the mobile station, and to reduce signal interference with the other subscribers simultaneously, thus improving the quality of signal link.
According to the existing protocol standard, in a dedicated up channel for a WCDMA and CDMA2000 system, a commonly used data structure is that a control channel DPCCH (Digital Processing Control Channel) and a service channel DPDCH (Digital Processing Data Channel) transmit in parallel through orthogonally scrambled coding. The DPCCH channel is operative to have an intermittent pilot (or guiding frequency) format (see FIG. 1) with a TDMA address in a slot including a pilot sign and a control message, and with BPSK (Binary Phase-Shifted Keying) modulated data.
For a transmit signal with the above data structure, a demodulation technique plays an important role in how to correctly and effectively recover or restore the original transmit signal at a receive terminal. A more effective correlation demodulation is that a received signal is multiplied by a detected reference signal to remove the effect of a fading channel in radio transmission, such as the deviation of phase and/or magnitude of a transmit signal. The reference signal is an estimation value of a signal that experiences the channel fading. Generally, link capacity is inversely proportional to SNR (Signal-to-Noise Ratio) of a receive signal receiver that needs to reach certain performance. For a signal experiencing channel fading, a correlation detection is a suitable detection scheme for a CDMA system because a correlation detection requires less SNR than an uncorrelation detection. Thus, accurate estimation of magnitude and phase of a fading channel is needed under a condition of multi-path channel with rapid channel fading. A lot of methods have been proposed including simple usage of a pilot message in a pilot field to interpolate a channel fading value of an outgoing data field; adapted forward prediction by using a data field and a pilot field simultaneously; or a lattice filter by using forward and backward predictions to estimate a channel and so on. These methods can obtain good results under a certain environment. However, in addition to satisfying the requirement of large data traffic transmission, a lot of data services of the third generation of mobile communication system must be suited for a large range of moving speed change, for example, from still to 500 km/h. It is difficult for the existing channel estimation methods to meet the requirement of communication quality for a system in various conditions.